Manufacture of radio-isotopes



M. V. BODNARESCU 3,396,077

MANUFACTURE OF RADIO-ISOIOPES Aug. 6, 1968 Filed Sept. 14, 1966 2Sheets-Sheet 1 FiG. w Pies INVENTOR Musuf V. BODNARESCU ATTORNEY 2Sheets-Sheet 2 M. v. BODNARESCU MANUFACTURE OF RADIO-I SOTOPES 1 ElliAug. 6, 1968 Filed Sept.

c RS1 A E/ NA R E 0 V T D T 0 A 91 0V 6 V, '0 F M 4 Claims. (in. 176-15)10 ABSTRACT OF THE DISCLOSURE An irradiation capsule for use in acontrol rod of a nuclear reactor for the production of radioisotopes inepithermal and fast neutrons. The capsule comprises an outer tubularshield in which a target material is enclosed, the shield being closedat the ends thereof by stoppers. The lower stopper is provided withspacing members to center the capsule within the control rod and havethem rest on a shoulder formed at the lower end of the control rod.These spacing members are retractable within the periphery of the lowerstopper to allow removal of the capsule from the control rods. A pair oftelescopic tubes are provided at the top of the capsule, the lower tubebeing secured to the upper stopper and the upper tube being providedwith laterally projecting spacers intended to be pressed against ashoulder at the upper end of the control rod by means of a Springcooperatively mounted on the two telescopic tubes to move them away fromone another.

This application is a continuation-in-part undiminished of my copendingapplication U.S. Ser. No. 404,537 (Series of 1960), filed Oct. 16, 1964,now abandoned.

This invention relates to a device for the production of radio-isotopes,particularly those radio-isotopes the nuclei or intermediate nuclei ofwhich are consumed too much by fission when exposed to normal in-pileirradiation.

In numerous fields such like medicine, metallurgy, petroleum industries,textile industries, the increasing importance of radio-isotopes is wellknown as is their use as a source of compact energy, particulary usefulin cases where weight and volume are strictly limited.

These radio-isotopes are generally produced by irradiation inside anuclear reactor: a material subjected to an appropriate neutronicbombardment undergoes changes which make it radioactive, that is,transform it into a radio-isotope.

This irradiation is made to take place either in the fuel channels ofthe reactor or in special easily accessible channels conceived forirradiation of encapsuled material.

Some radio-isotopes are generated by (n,'y)-reactions. These reactionslead to desired neutron capture, accompanied by the undesired competingfission process.

Especially when a nuclear reaction of higher order is in the case, whereseveral neutrons (one after the other) have to be captured from thestarting nucleus on, it is extremely important to keep the ratio of thecross sections capture/fission as high as possible. The increase of fluxintensity with otherwise equal neutron energy spectrum does not changethis ratio. Then an advantage exists only where the undesired sidereaction is a natural decay process and not a fission process.

If one wants to influence the ratio capture/fission there is left onlythe way to choose an appropriate neutron spectrum.

For many nuclides the ratio capture/fission for irradianited tatcsPatent 3,396,077 Patented Aug. 6, 1968 tion in an epithermal neutronflux is increased by increasing neutron capture or by diminishingneutron-induced fission.

There may be chosen two possibilities for the irradiation of suchnuclides in epithermal and fast neutron fluxes:

(a) An in-pile region with high epithermal neutron flux-component andnon-exclusion of the thermal flux, and

(b) Exclusion of thermal neutron flux by an appropriate absorber andirradiation in a purely epithermal neutron flux.

A simple way for irradiation with epithermal neutrons according topossibility (b) is the irradiation of the target substance in a cadmiumcapsule within the irradiation channel of a nuclear reactor. But aconsiderable disadvantage of this way is the unfavorable deformation ofneutron flux distribution, whereby the neutron flux is diminished notonly around the target itself but also no further irradiation may beundertaken in the vicinity of the irradiation channel in question. Thefuel in this zone stays cold, no neutrons are produced and neutroneconomy is suffering badly.

It is an object of the present invention to provide a device widelyadaptable to produce radio-isotopes in epithermal and fast neutronfluxes without the aid of means normally foreseen for the production ofradio-isotopes like irradiation of capsules in irradiation channels,fuel elements or special radio-isotope production assemblies. It is afurther object of the invention to provide a device for a high-yieldproduction of radio-isotopes, the yield of which is low when produced bynormal in-pile irradiation, due to fission induced nuclei consumingreactions.

Other objects and advantages of the invention will become apparent tothose skilled in the art upon becoming familiar with the followingdescription and claims, reference being made to the appended drawings.

This invention is based upon the idea to use and utilize thethermal-neutron-void interior of the absorber part of control means innormal nuclear reactor control systems for the production ofradio-isotopes.

For the purpose of the specification and claims reactor control meansare defined as means to control neutron density of a nuclear reactor bypositioning mechanically absorber material or control material (nuclearfuels excepted) in the form of control rods, shim rods or shim bodies,in general: movable absorbers for thermal neutrons as they are used inreactor engineering for reactor control.

Reactor control means here do not comprise other means having aninfluence on the control of a nuclear reactor like fuel elements orreflectors being varied in their position, or targets to be irradiatedin irradiation or fuel channels of a reactor.

FIGURE 1 of the appended drawings is a schematic sectional view througha multi-layer tube serving as a control rod.

FIGURE 2 shows a schematic cross section through a multi-layer tubesimilar to the one shown in FIG. 1;

FIGURE 3 shows a modified absorber rod;

FIGURE 4 shows an in-pile arrangement of absorber rods as shown in FIG.3;

FIGURE 5 shows an absorber plate;

FIGURE 6 shows another absorber plate;

FIGURE 7 is a diagrammatic sectional view of a normal control rod;

FIGURE 8 is a fragmentary sectional view of a capsule to be disposedinside the control rod FIG. 7 as shown in FIG. 9.

With reference to FIG. 1 the tube is canned on the inside d and theoutside a. Behind the outside canning layer a is contained a thermalneutron absorber b serving as a transparent filter for epithermal andfast neutrons which are designed to impinge on the target material inorder to produce radio-isotopes therefrom.

With reference to FIG. 2, a and e are outside and inside canningrespectively, c being the thermal neutron absorber, d the targetmaterial to be irradiated, and b a layer of cobalt balls to beirradiated, as is already known. In FIG. 3, a is the outside canninglayer (no inside canning being provided), b is a thermal neutronabsorber and c is the target material contained in an axial cavity.

With reference to FIG. 4, a, b, and 0 have the same meaning as thatindicated in reference to FIG. 3. In FIG. 5, a is the canning sheath andb either a mixture or an alloy or chemical compound of absorber materialand target material.

As to FIG. 6, a indicates the canning sheath, [2 the thermal neutronabsorber layer and c an axial cavity filled with target material.

FIGS. 1 to 6 represent modifications of a prototypical embodiment of theinvention, reference being had mainly to FIGS. 7 to 9, where FIG. 7shows a sectional view of a typical control rod, particularly thein-core and near-core parts of it, the parts carrying drive, clutch, andpositioning mechanisms being omitted as well as the shock absorber part.

In FIGS. 7 and 8 equal numbers design equal parts: 1 is an outercanning, provided with holes allowing passage of cooling fluid, withinthe outer canning is mounted slideably an elongated tube 3 provided withlower and upper connecting parts 4 and 5, respectively, 3 serving as acanning tube for the tube from absorber material 6, e.g., cadmium,normally sheathed with aluminium. Through the central space 7 and thecircular clearance 8 cooling fluid 9 may pass and traverse also theconcentric tubes of solid moderator material 10, as under reactorconditions considerable quantities of heat are produced by theabsorption of thermal neutrons in the absorber material. Now in carryingout a preferred embodiment of the invention the irradiation capsule 11shown in FIG. 8 is disposed in the central space 7 of FIG. 7 as shown inFIG. 9. The capsule as shown in FIG. 8 is composed of a capsule tube 12provided with an upper, 13, and a lower, 14, stopper welded on the tube12. The upper stopper 13 is provided with a narrow aperture 15 servingfor filling tube 12 with a protective gas, which aperture 13 is closedbefore inreactor use. The lower stopper 14 is conceived in a manner topermit spring 16 supported positioning of the laterally slideable member17 on the shoulder 18 as shown in FIG. 9. The lower stopper 14 isprovided with a pressure equalization aperture 19. When pressure isexerted mechanically from below on member 20, members 17 are releasedand the capsule 11 may be withdrawn from above after removal of theupper connection parts and what is above. More specifically, the spacingmembers 17 are pressed resiliently against the inner wall of theabsorber tube, pressure being exerted by the spring 16, transmitted overmember 20. When pressure is exerted in axial direction on member 17,from above, the latter rotates clockwise towards the axis of theassembly and within the periphery of the lower stopper 14, allowing thecapsule to be used in a narrow channel.

The upper stopper 13 carries, screwed upon it, a guiding tube 21provided with elongated holes 22 permitting cooling fluid flowdistribution inside of this guiding tube. On its upper end the tube 21carries a terminal member 23, which is positioned, supported by spring24, against a screwable nose 25. The terminal member 23 is carrying atriple flange 26, supported circular disc 27, which is positionedagainst the upper connection part 5 (FIG. 9) and provided with a centralhole 28, serving as cooling fluid inlet. The target material is filledinto the capsule 11, the capsule closed with the stopper 13 by welding,the welded joints examined by X-radiography, the atmosphere inside thecapsule 11 replaced by protective gas atmosphere through 15 and closedby welding. Then the guiding tube 21 is screwed upon the stopper 13 andthe whole capsule positioned within the control rod, the connectionpiece 5 introduced together with drive, clutch, and positioningmechanisms usually foreseen for the control rod.

An example, explaining the advantages of the invention is the productionof Beryllium-10 carrier free through irradiation of Boron-l0 in theepithermal-fast flux of a reactor.

If the thermal neutron flux component was not eliminated, Boron-1O witha cross section of 3820 barn would be consumed by the thermal neutronsthrough a nuclear reaction furnishing helium and lithium. For thatreason, when Boron-10 is irradiated in the device according to theinvention, Beryllium-10 is furnished through a (n,p)-reaction. TheBeryllium-l0 is carrierfree, when there was no beryllium contained inthe target.

Another example is the irradiation according to the invention ofCurium-244 for the intermediate production of curium-246, from which, asa new starting material through irradiation in the total flux of areactor, one obtains in nuclear reactions leading over curium andberkelium isotopes the appreciated californium.

Here, the first step Cm-244 Cm-246 has to be conducted under eliminationof the thermal neutron flux in order to influence the unfavorablecapture/ fission ratio of curium-245. The essential advantage of thedevice lies in the fact that no conventional irradiation device likeirradiation channels are needed but only better use is made of reactorcontrol means already present in any reactor whatever may be itsconception. Through the device according to the invention normal fluxdistribution and flux intensity is not in the slightest way influenced,as is the case when working with thermal neutron absorbing capsules inirradiation channels.

The most elegant case is that explained as a prototypical embodiment ofthe invention: a capsule placed within the hollow thermal neutronabsorbing cadmium tube of a conventional control rod.

Yet the control means for nuclear reactors are often different of thetype shown: there are massive bars in rod form, there are control bodieshaving other forms like balls, which are technologically controlled andmoved automatically to control neutron flux distribution and neutronflux density. So the device according to the invention may haveditferent geometrical forms, the axial cavity inside a Cadmium likeabsorber material may be filled with target material as is shown in FIG.3. Varying forms are shown too in FIG. 6, where the section is ofparallel-epipedic form, and in FIG. 5 where an elongated section form isfilled with a powder mixture or an alloy or a chemical compound ofcontrol material and target material.

Thus b in FIG. 5 may represent a powder mixture of Cadmium and Boron-10,or a Cadmium-Curium-244 alloy.

An advantageous modification of the invention is the exposition of thetarget material to be irradiated to the epithermal and fast neutron fluxbehind the shield of control material within technologically employednuclear reactor control means, such target material being wrapped in oralloyed with material serving as a filter for the epithermal and fastneutrons impinging on them and allowing only a certain desired energyrange of neutrons to pass for which the capture cross section of thetarget material is optimal.

Such filter substances may be chosen from neutron spectrum cataloguesaccording to the problem to be solved. The cavities of control rods mayalso be lined with filter materials. As in the case of the capsuleirradiated in a Water cooled cavity inside of a reactor control rodepithermal and fast neutrons are moderated by the cooling Water, it isadvisable to line the capsule with the control material employed in therod or another one.

The advantages of the invention are remarkable from the point of view ofeconomics, particulary in the case of the transplutoniums, i.e. elementshaving an atomic number greater than 94.

Literature: Reactor Handbook 2nd edition, volume I-Materials; volumeIV-Engineering.

What I claim is:

1. In control means of nuclear reactors, the combination for theproduction of radioisotopes in epithermal and fast neutrons, comprising:

(a) a hollow neutron absorbing control rod and shoulder means at theends of said rod;

(-b) an irradiation capsule within said rod, between the ends thereof,of a cross-section to be loosely received therein; said capsulecomprising:

an outer tubular shield, a target material enclosed by said shield, andstoppers closing the ends of said tubular shield;

(c) spacing members mounted on the lower one of said shield stoppers,said spacing members laterally projecting from said lower stopper andshield to rest against the lower one of said control rod shoulder means;

(d) means allowing said spacing members to retract Within the peripheryof said lower stopper to allow removal of said capsule from said controlrod;

(e) guiding means for said capsule formed of telescopic upper and lowermembers, said lower member secured to the upper one of said shieldstoppers and said upper member provided with spacing means to centersaid capsule in said control rod, and

(f) resilient means between said telescopic members to press saidspacing means against the upper one of said control rod shoulder means.

2. A combination as claimed in claim 1 wherein said shield is lined withneutron filter material capable of selecting optimal neutron energyranges from the epithermal and fast neutrons to impinge on the targetmaterial.

3. A combination as claimed in claim 2 wherein said tanget material is apowder mixture.

4. A combination as claimed in claim 2 wherein said target material isan alloy.

References Cited UNITED STATES PATENTS 2,743,226 4/1956 Newson 176-153,042,598 7/1962 Crowther 176-93 3,049,484 8/1962 Zinn 176-15 3,052,6169/1962 Graham 176-21 3,103,479 9/1963 Ransohoff 176-93 3,138,534 6/1964Frisch et a1. 176-15 3,250,729 5/1966 Petzow et al. 176-93 3,255,0926/1966 Dee 176-93 3,269,915 8/ 1966 Ransohoff et a1. 176-16 FOREIGNPATENTS 934,343 8/1963 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

H. E. BEHREND, Assistant Examiner.

